This invention relates to a vehicular hands-free telephone system mounted in a vehicle such as an automobile.
FIG. 10 shows the mounting in a passenger vehicle of a prior-art hands-free telephone apparatus. In the drawing, the reference numeral 1 denotes the passenger vehicle, which has a transceiver 2 mounted in its trunk. An antenna 3 is mounted on the roof of the passenger vehicle 1 and connected to the transceiver 2 by an antenna cable 3a. The numeral 4 denotes an automobile battery which is connected to the transceiver 2 by a power cable 4a and supplies power to the transceiver 2. A phone base 5 is installed near the driver's seat in the passenger vehicle 1, and is connected to the transceiver 2 by a control cable 5a. The top of the phone base 5 is provided with a handset cradle, on which a handset 6 is placed. The handset 6 is connected to the phone base 5 by a curled cord 6a. A speaker 7 for hands-free communication is also installed in the cabin of the passenger vehicle 1 and is connected to the phone base 5 by a speaker cable 7a. A microphone 9 is mounted on the sun visor 8 and is connected to the phone base 5 by a microphone cable 9a.
FIG. 11 is a perspective drawing providing a more detailed view of the mounting in the vicinity of the driver's seat described in FIG. 10.
FIG. 12 is a block diagram of the prior-art hands-free telephone apparatus, in which the reference numeral 11 denotes a regulated power-supply circuit, 12 is an analog switch for switching between a handset mode in which the conversation is carried via the handset 6 and a hands-free mode in which the conversation is carried via the microphone 9 and speaker 7, 13 is a hook switch for switching in accordance with the placement of handset 6, and 14 is a logic circuit for decoding the status of keys 61 in the handset 6, sending display commands to a display device 62, exchanging display data with the transceiver 2, processing the data, monitoring the state of the hook switch 13, and controlling the switching of the aforementioned analog switch 12. The reference numeral 15 denotes a transmit amplifier for amplifying the signal received from the mouthpiece 63 in the handset 6 to a required level, and 16 is an interface circuit for amplification or attenuation to a level optimal for the operation of the earpiece 64 in the handset 6. The reference numeral 17 denotes a transmission control circuit for comparing the transmit level of the outgoing voice signal from the microphone 9 with the level of the incoming voice signal in the hands-free mode and controlling howling and abnormal fluctuations in voice level to optimize the hands-free telephone operation. The operation of the transmission control circuit 17 is well known and will not be further described herein.
Next the operation of the apparatus will be described. The basic operation of the transceiver 2 of the automobile telephone system is general knowledge so it will not be described here, but it will be assumed that power and the incoming voice signal are transferred from the transceiver 2 to the phone base 5, the outgoing voice signal is transferred from the phone base 5 to the transceiver 2, and the transceiver 2 and the phone base 5 exchange data, if necessary.
When the handset 6 is resting on the phone base 5, if a key 61 on the handset 6 is pressed to designate the hands-free mode, the logic circuit 14 reads the status of the hook switch 13 and the key 61, indicating that the handset 6 is resting on the phone base 5 and that the hands-free mode is designated, and sets switches in the analog switch 12 to the hands-free mode in which the outgoing voice signal armature is connected to the contact 12a and the incoming voice signal armature is connected to the contact 12c. The hands-free microphone 9 is connected by the microphone cable 9a so that it receives power from the phone base 5 and sends its output to the phone base 5. To prevent howling the speaker 7 is located as far as possible from the microphone 9, although still within easy hearing range of the person using the telephone. More specifically, to prevent the sound output from the speaker 7 from entering the microphone 9 directly at a high volume level, the speaker 7 is positioned so that the output level will at least be lower than the level of voice input to the microphone 9. For hands-free operation it is therefore desirable to have the microphone 9 located near the hands-free person's mouth. Mounting the microphone 9 on the sun visor 8 is already widely employed as a means of bringing the microphone 9 as close as possible to hands-free person's speaker's mouth. The voice signal picked up by the microphone is sent to the transmission control circuit 17, which compares it with the incoming voice signal from the transceiver 2 and applies control so that the gain of the closed loop created when the acoustic output of the speaker 7 is picked up by the microphone 9 and transmitted to the other party's speaker, then is picked up by the other party's microphone and returns to the speaker 7 is less than unity, so that howling does not occur. For example, the transmission control circuit 17 may apply control to optimize the operation of the hands-free mode by attenuating the receive level when the transmit level is higher and attenuating the transmit level when the receive level is higher.
Thus for the hands-free operation to function smoothly it is essential that the hands-free microphone 9 be placed as close as possible to the hands-free person's mouth.
An example of prior art differing from that described above is given in Japanese Patent Laid-open No. 203359/1982, in which the objective of situating the microphone as close as possible to the hands-free person's mouth is accomplished by providing a transducer for converting an acoustic signal to an electrical signal in the frame of a pair of glasses.
Problems occurring in a prior-art hands-free telephone apparatus configured as described above include, in the case of apparatus having a microphone 9 mounted in the frame of a pair of glasses as in Japanese Patent Laid-open No. 203359/1982, high cost and the inconvenience of having to wear and operate the apparatus, and in the case of apparatus in which the microphone 9 is provided in the sun visor 8 as in FIGS. 6, 7, and 8, the trouble of having to install a microphone cable to connect the microphone 9 to the phone base 5, the possibility of entanglement because this cable runs partly under the driver's feet, and the aesthetic problem that the cable is exposed to view.